I think Mixon, Bell is a great 1/2 keeper pending Bell comes around and you grab Conner as insurance.
If Bell’s holdout looks serious (I don’t think he’s reported as of yet and they had a meeting at 9am) and you get a first round pick back by throwing him back to the draft pool and there’s some options on the board, I’d consider going that route: keep Mixon and Gronk and grab someone with that pick that’s available - Barkley, Kamara, Howard, Fournette etc.
I understand that Bell’s a league winner, but this whole scenario is such a headache. If he ends up holding out, it’s really going to suck and you’ve sunk a first round keeper for nada. If you can get his insurance with Conner and play the long game, it could really pay out - there’s risk reward with this.
TL;DR- Mixon and Bell, just make sure you grab Conner in the draft to lock up that backfield.
Hopefully we hear a definitive answer as to what’s going on with Bell before your draft/keeper deadline etc.